Various systems have been proposed and are in use for scrambling television signals to obtain the secure transmission of video information. One such scrambling technique involves what has been termed "block shuffling". In block shuffling the television field consisting of contiguous video lines is divided into several blocks or groups of video lines, such as for instance eight lines in each block. The video lines within each block are then randomly shuffled or scrambled so that the original line sequence has been changed to a new scrambled line sequence within each block. The scrambled video signals are then transmitted to a receiver along with data relating to a code corresponding to the order of the randomly shuffled lines in each block.
At a receiver having a corresponding decoder, the shuffled video lines in each block are then returned to their original sequence so that the video display of each block in sequence reforms the original field. Of course, if the receiver does not contain a suitable decoder, the displayed video signals will be generally unintelligible and the information masked since the original line sequence of the displayed field has been altered.
Reference may be made for instance to Block, et al., U.S. Pat. No. 4,405,942 showing a video scrambling system in which the sequence of individual horizontal lines of video information are rearranged in a determinable manner relative to the normal sequence of lines in a raster scan field of video information. In this patent, line shuffling occurs over the entire field. For instance one or more of the original last lines of the field may appear as the first lines of the scrambled field; the original first lines may appear as the last lines of the scrambled field; and an original line can be scrambled to any position within the normal 262 line field.
In the scrambling technique shown in this patent, the video lines are stored in their original sequence in a first storage device. An address generator is encoded to control the sequence of addresses generated so that the video lines are read from the first storage device in a sequence differing from the original sequence and the scrambled sequence is transferred to a second storage device. A coded identification signal is also supplied for transmission with the video lines as they are read from storage. This patent shows a pseudo-random code generator that produces a multiple bit code word at the end of each video line which may be used to clock the video lines out of the first storage device on a line-by-line basis in a pseudo-random fashion. At a decoding station the original line sequence is reconstituted in response to the coded signal and the normal sync and blanking signals.
Reference may also be made to Kupnicki, et al., U.S. Pat. No. 4,742,544 which shows a television scrambling system where a block of a given number of video lines in a television field is selected and all of the lines within that block are randomly intermixed or scrambled to change the original line sequence. A block of four video lines are written into storage in accordance with a random four line pattern set by a corresponding code or key. As an example, if the random code consists of four numbers 3-1-2-4 forming the storage address, the sequence of video lines forming this block will be entered into storage with the first video line entered into memory position 3; the second video line entered into memory position i; the third video line entered into memory position 2; and the fourth video line entered into memory position 4.
The memory is then read sequentially so that this block of video lines is transmitted in a scrambled sequence of video lines 2-3-1-4. The random code pattern 3-1-2-4 is also transmitted in a coded data format along with the scrambled sequence of video lines.
At the receiver, the transmitted video lines for this block are read into storage in a sequential manner as the information is received so that video lines 2-3-1-4 are respectively stored in the four consecutive memory positions. The random code pattern 3-1-2-4 is recovered from the transmitted video signal and decoded so that the memory is addressed to read out the video lines from the memory in memory positions 3-1-2-4 in accordance with the random code. Thus, the video information read out of storage will be displayed in the original sequence for this block, i.e. video lines 1-2-3-4.
The shuffling of a group of video lines on a block-by-block basis where each block is less than an entire field is desirable in comparison to shuffling of the entire field in order to use smaller memories and lesser numbers of associated address generators and other required components. However, the probability that a particular line will be displaced an increasing distance from the original position during shuffling significantly depends on the size of the block. Thus for a small block of about eight video lines, the expected displacement during shuffling of any given line is restricted to the original eight positions. For instance, using probability theory, in shuffling a block of eight video lines the expected value of line displacement of the first line ca be calculated to be equal to 3.5 lines. The average expected value of line displacement (AEVD) for all lines in the block can also be computed as a figure of merit. In the instance of a block of eight lines, the AEVD equals 2.625 for normal block shuffling, i.e. random shuffling of a block of eight video lines.
The AEVD value can of course be increased by increasing the size of the block to include more video lines, such as 16 or 32 which is desirable. However, while the AEVD value always increases with block size, eventually a block size is reached which is undesirable as it contains a large number of video lines requiring a larger memory and more associated memory addressing components. In the extreme, of course, the "block" could consist of the entire field, which as indicated before is undesirable.
Accordingly, it is desired to provide a video scrambling technique using line shuffling of video lines within discrete blocks of video lines less than the entire field and wherein the AEVD value can be increased without the need to increase the block size of video lines.